The present invention is directed to processes for cleaning a solid surface and, in particular, processes for cleaning a part such as a metal workpiece, a printed circuit board and the like, utilizing an aqueous cleaning slurry.
Parts washers of various kinds are known to those skilled in the art as having great utility for mechanics and others working in a variety of occupations, particularly those working in industrial plants, maintenance and repair services, and the like. The parts washers referred to herein include soak tanks, so-called hot tanks, immersion type parts cleaners with or without air agitation, spray washers (continuous or batch) and ultrasonic baths. Generally, parts washers are used to remove all types of contaminants adhered to a solid surface such as metal including such contaminants as greases, cutting fluids, drawing fluids, machine oils, antirust oils such as cosmoline, carbonaceous soils, sebaceous soils, particulate matter, waxes, paraffins, used motor oil, fuels, etc.
Until recently, metal surfaces were cleaned of most oily and greasy contamination in such parts washers as referred to above by use of organic solvents. Existing solvents, with or without special additives, are adequate to achieve good cleaning of most dirty, greasy, metal parts. A great number of solvents have been employed to produce metallic surfaces free from contamination. These wash solvents generally include various hydrocarbons and halogenated hydrocarbons. The degree of success achieved with each of these wash solvents is generally dependent upon the degree of cleanliness required of the resultant surface.
Recently, however, the various hydrocarbon and halogenated hydrocarbon cleaning solvents previously employed have come under scrutiny in view of the materials employed, and in particular, the environmental, health and safety impacts from the usage of the various materials. Thus, although the halogenated hydrocarbon solvents such as chlorofluorocarbons (CFCs) and trichloromethane, methylene chloride and trichloroethane (methyl chloroform) are widely used in industry for cleaning, their safety, environmental and cost factors coupled with waste disposal problems are negative aspects in their usage. A world-wide and U.S. ban on most halogenated hydrocarbon solvents is soon in the offing by virtue of the Montreal Protocol, Clean Air Act and Executive and Departmental directives. The hydrocarbon solvents such as toluene and Stoddard solvent and like organic compounds such as ketones and alcohols on the other hand are generally flammable, have high volatility and dubious ability to be recycled for continuous use. These, plus unfavorable safety, environmental and cost factors, put this group of solvents in a category which is unattractive for practical consideration. Most useful organic solvents are classified as volatile organic compounds (VOCs) which pollute the atmosphere, promote formation of toxic ozone at ground level, and add to the inventory of greenhouse gases.
Alternative processes have been developed for removing contaminants from solid surfaces other than the use of organic solvents. Among such alternatives include blast cleaning processes in which an abrasive is directed against a contaminated surface by means of a pressurized fluid stream such as air or water. Unfortunately, hard abrasives such as sand, aluminum oxide, steel shot and the like may actually harm the underlying surface during treatment. Not only metal surfaces but nonmetallic surfaces such as glass or silicone such as used to form printed circuit boards have also been cleaned of contaminants comprising oils, greases, waxes, fluxes and the like using a blast cleaning process in which the blast medium comprises fine particles of dry ice. While such methods are readily useful in affecting the cleaning of parts having relatively uniform shapes or flat boards, such methods cannot easily clean the whole surface of a printed circuit board on which many electronic parts are mounted, or parts having complicated structures or containing holes since it is not easy to direct the motion of the blast medium to effectively reach such surfaces.
Water blasting systems have been used in which the water blast stream contains insoluble abrasive particles. Besides causing possible harm to the surface of the substrate due to the abrasive action of the hard particles as described above, such water blasting process often leaves residues of fine insoluble particulates on the substrate. If the substrate is of complex configuration, contains holes or includes precision moving parts, it is obvious that the presence of the particulate residues can degrade the usefulness of the part and possibly harm the part during use thereof. It is known, however, to utilize water soluble abrasives in the water blast stream. While the use of the water soluble abrasives eliminates the problem of insoluble residues remaining on the part inasmuch as the parts can be washed with water to dissolve any remaining particulate abrasive, the wet blast process itself like the purely dry blast process is equipment intensive requiring containment means such as blast cabinets or blast room installation, compression equipment, supply hoppers, hoses and nozzles and means to accurately dispense the abrasive into the pressurized fluid stream and is labor intensive. Further, if water soluble abrasives are used in a pressurized water stream, the problem of abrasive solubility and maintaining abrasive particle size to clean effectively poses further problems. This latter problem has been overcome by using a fluid stream comprising a saturated solution to limit abrasive solubility in the fluid stream and maintain abrasive particle integrity. Copending, commonly assigned U.S. Ser. No. 105,406, filed Aug. 12, 1993, now U.S. Pat. No. 5,384,990 and Continuation-in-part application (Docket 23693A) filed concurrently herewith describe water blasting with a water soluble abrasive contained in a saturated solution.
Another alternative cleaning process provided to eliminate the various negative aspects of the known organic solvent washing and degreasing systems, involves the use of aqueous detergent systems. Unfortunately, aqueous cleaning systems are not without their own problems. For example, certain of the aqueous cleaners are exceedingly alkaline having pHs of 13 and above such as sodium hydroxide or include organic solvents such as alkanolamine, ethers, alcohols, glycols, ketones and the like. Besides being highly corrosive, the exceedingly high alkaline aqueous solutions are highly toxic and can be dangerous to handle requiring extreme safety measures to avoid contact with skin. Organic solvent-containing aqueous cleaners present the problems regarding toxicity, volatility or the environment as expressed previously. On the other hand, it is most difficult to obtain an aqueous detersive solution at moderate pH which is effective in removing greases and oils and other hard to clean soils and the like and which would not be corrosive such as to metal substrates. Further, to provide effective cleaning using aqueous detersive solutions at moderate pH, it is often necessary to utilize high temperatures well upwards of 150.degree. F. and typically approaching 170.degree.-195.degree. F. Obviously, the use of such elevated temperatures, long residence times and additional vigorous mechanical cleaning such as brushing, spraying, blasting adds costs to the cleaning method.
Still another alternative to the mere use of organic solvents is described in U.S. Pat. No. 5,226,969. In this patent, there is disclosed an apparatus for cleaning a solid surface which comprises a cleaning bath containing a cleaning liquid which contains insoluble particles and further includes means for bringing the cleaning liquid into an eddy-flow state. The insoluble particles may be particles such as glass, silicon, alumina, calcium carbonate and cerium oxide. The liquid can include pure or deionized water, a detergent and an organic solvent. The means for bringing the cleaning liquid into an eddy-flow state may be a stirring apparatus or a vibration generating apparatus and the like. The problem with this cleaning procedure is the use of insoluble abrasives. Thus, abrasives which are very hard, can actually harm the underlying surface which is to be cleaned. Further, as previously stated, particles of the insoluble abrasive may remain in the crevices of a substrate which has a complicated structure. This residue of abrasive particles may actually cause damage to the substrate being cleaned during use. For example, a precision part or a printed circuit board may actually fail during use due to the pressure of a residue of insoluble abrasives.
An object of the present invention is to provide a cleaning system which overcomes the environmental, safety and health hazards found in purely organic solvent cleaning systems.
Another object of the present invention is to provide a cleaning system which is safe to use, not a hazard to the environment and requires reduced labor, equipment and energy costs relative to the concerns and costs associated with previous organic solvent cleaning, aqueous detergent cleaning and abrasive blast cleaning processes.
Another object of the present invention is to provide a cleaning system which can readily clean complicated parts and surfaces which have variegated structure, holes, slots and the like.
Still another object of the present invention is to provide an aqueous based cleaning system which can be used to effectively clean contaminants from surfaces at neutral to moderate pH and which any residues of the cleaning system can be readily removed with an aqueous rinse.
These and other objects of the invention can be readily discerned from a description of the invention which is set forth in more detail below.